[llvm-dwarfdump][Statistics] Distinguish functions/variables with same name across different CUs

Different variables and functions might have the same name in different CU.
To calculate 'Availability' metric more accurate (i.e. to avoid getting
availability above 100%), we need to have some additional logic to
distinguish between them.

The patch introduces a DIE identifier that consists of a function/variable name
and declaration information: a filename and a line number. This allows
distinguishing different functions/variables (different means declared in
different files/lines) with the same name, keeping duplicates counted
as duplicates.

Reviewed by: aprantl, djtodoro

Differential Revision: https://reviews.llvm.org/D72797